How to open all fonts at once?
Use python.

from bpy import ops
from glob import glob
from os import chdir, path

directory = "C:\\Windows\\Fonts"

chdir(directory)
for file in glob("*.ttf") + glob("*.otf"):
    ops.font.open(filepath = path.join(directory, file), relative_path=False)


Replace the directory variable with the folder path, which contains your font files. Escape \ with \.

Paste the script in the text editor and press Run Script.